Is there a method like JavaScript's substr in Rust?

Question

I looked at the Rust docs for String but I can\'t find a way to extract a substring.

Is there a method like JavaScript\'s substr in Rust? If not, how would you implement

Answer 1

The solution given by oli_obk does not handle last index of string slice. It can be fixed with .chain(once(s.len())).

Here function substr implements a substring slice with error handling. If invalid index is passed to function, then a valid part of string slice is returned with Err-variant. All corner cases should be handled correctly.

fn substr(s: &str, begin: usize, length: Option) -> Result<&str, &str> {
    use std::iter::once;
    let mut itr = s.char_indices().map(|(n, _)| n).chain(once(s.len()));
    let beg = itr.nth(begin);
    if beg.is_none() {
        return Err("");
    } else if length == Some(0) {
        return Ok("");
    }
    let end = length.map_or(Some(s.len()), |l| itr.nth(l-1));
    if let Some(end) = end {
        return Ok(&s[beg.unwrap()..end]);
    } else {
        return Err(&s[beg.unwrap()..s.len()]);
    }
}
let s = "abc